One problem of existing drug delivery devices, which could be, for example, syringes, especially safety syringes, having a retractable needle, is to connect the plunger or the needle assembly so that the needle assembly with the needle could be drawn back into the body unit. The needle assembly and the needle should be drawn back into the body unit so that nobody can be injured by the needle after the use of the drug delivery device.
Some of the drug delivery devices, especially safety syringes having a retractable needle, have a plunger and a needle assembly whereby the plunger and the needle assembly are formed so that they can get into a mechanical connection with each other. They may be formed in a complex way so that if the plunger is pressed onto the needle assembly, the two parts snap into each other like in a key-lock-mechanism.